A kinetic study of the H-abstraction reaction from cyclic and acyclic alkylarene substrates by the nitroxyl radical (dubbed BTNO) of 1-hydroxy-benzotriazole (HBT) has been carried out in MeCN solution at 25 8C. BTNO was generated from one-electron oxidation of HBT by cerium(IV) ammonium nitrate. The H-abstraction reactivity measured with the cyclic alkylarenes is invariably higher than that with the acyclic counterparts. This is explained as the contribution of hyperconjugation between the aromatic p-system and the scissile benzylicC—Hbond of the substrate, which weakens the C—H bond in the transition state and promotes its cleavage. Stereoelectronic considerations enable to appreciate why the weakening effect is more pronounced in the cyclic system than in the acyclic counterpart, thereby justifying the higher reactivity of the former. Evidence for the intervention of stereoelectronic effects is embodied by the dissociation energies of the C—H bonds, having always lower values for the cyclic substrates investigated.
Stereoelectronic effects in radical processes / Paolo, Brandi; Galli, Carlo; Gentili, Patrizia. - In: JOURNAL OF PHYSICAL ORGANIC CHEMISTRY. - ISSN 0894-3230. - 19:8-9(2006), pp. 552-554. (Intervento presentato al convegno 10th European Symposium on Organic Reactivity ( ESOR-10) tenutosi a Rome, ITALY nel JUL, 2005) [10.1002/poc.1054].
Stereoelectronic effects in radical processes
GALLI, Carlo;GENTILI, Patrizia
2006
Abstract
A kinetic study of the H-abstraction reaction from cyclic and acyclic alkylarene substrates by the nitroxyl radical (dubbed BTNO) of 1-hydroxy-benzotriazole (HBT) has been carried out in MeCN solution at 25 8C. BTNO was generated from one-electron oxidation of HBT by cerium(IV) ammonium nitrate. The H-abstraction reactivity measured with the cyclic alkylarenes is invariably higher than that with the acyclic counterparts. This is explained as the contribution of hyperconjugation between the aromatic p-system and the scissile benzylicC—Hbond of the substrate, which weakens the C—H bond in the transition state and promotes its cleavage. Stereoelectronic considerations enable to appreciate why the weakening effect is more pronounced in the cyclic system than in the acyclic counterpart, thereby justifying the higher reactivity of the former. Evidence for the intervention of stereoelectronic effects is embodied by the dissociation energies of the C—H bonds, having always lower values for the cyclic substrates investigated.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
